Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Assets
Available-for-sale investment securities, at fair value	$ 996,984	$ 820,648
Stock in Federal Home Loan Bank, at cost	11,218	10,678
Net loans	4,675,901	4,565,781
Loans held for sale, at lower of cost or fair value	26,743	4,631
Other	459,187	480,457
Goodwill	82,190	82,190
Total assets	12,198,985	11,782,018
Liabilities
Other	489,466	471,828
Total liabilities	10,096,304	9,820,085
Commitments and contingencies
Equity [Abstract]
Retained earnings	427,990	324,766
Accumulated other comprehensive income (loss), net of taxes:
Accumulated other comprehensive loss, net of tax benefits	(17,023)	(26,262)
Total shareholders’ equity	2,068,388	1,927,640	$ 1,921,001	$ 1,790,573
Total liabilities and shareholders’ equity	12,198,985	11,782,018
Other assets
Premises and equipment, net	4,532,556	4,377,658
Total Other Assets	459,187	480,457
Other liabilities
Total other liabilities	489,466	471,828
Balance Sheet related disclosures
Securities sold under agreements to repurchase	165,000	229,000
American Savings Bank (ASB)
Assets
Cash and due from banks	109,591	127,201
Interest-bearing deposits	103,989	93,680
Available-for-sale investment securities, at fair value	996,984	820,648
Stock in Federal Home Loan Bank, at cost	11,218	10,678
Loans receivable held for investment	4,734,638	4,615,819
Allowance for loan losses	(58,737)	(50,038)
Net loans	4,675,901	4,565,781
Loans held for sale, at lower of cost or fair value	26,743	4,631
Other	330,054	309,946
Goodwill	82,190	82,190
Total assets	6,336,670	6,014,755
Liabilities
Deposit liabilities—noninterest-bearing	1,570,613	1,520,374
Deposit liabilities—interest-bearing	3,810,108	3,504,880
Other borrowings	265,388	328,582
Other	106,396	101,029
Total liabilities	5,752,505	5,454,865
Commitments and contingencies
Equity [Abstract]
Common stock	1	1
Additional paid in capital	342,234	340,496
Retained earnings	250,726	236,664
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized gains (losses) on securities	5,965	(1,872)
Retirement benefit plans	(14,761)	(15,399)
Accumulated other comprehensive loss, net of tax benefits	(8,796)	(17,271)
Total shareholders’ equity	584,165	559,890
Total liabilities and shareholders’ equity	6,336,670	6,014,755
Other assets
Bank-owned life insurance	141,262	138,139
Premises and equipment, net	91,354	88,077
Prepaid expenses	4,072	3,550
Accrued interest receivable	15,489	15,192
Mortgage-servicing rights	9,191	8,884
Low-income housing equity investments	48,474	37,793
Real estate acquired in settlement of loans, net	219	1,030
Other	19,993	17,281
Total Other Assets	330,054	309,946
Other liabilities
Accrued expenses	37,671	30,705
Federal and state income taxes payable	13,971	13,448
Cashier’s checks	24,923	21,768
Advance payments by borrowers	5,531	10,311
Other	24,300	24,797
Total other liabilities	106,396	101,029
Balance Sheet related disclosures
Securities sold under agreements to repurchase	165,000	229,000
Advances from Federal Home Loan Bank	$ 100,000	$ 100,000